Distributed network storage system with virtualization

ABSTRACT

The present invention is directed to a data storage system for use in achieving distributed data storage over a computer network. One embodiment of the data storage system comprises a storage server system that is comprised of one or more storage servers that each provide data storage, a management server system that is comprised of one or more management servers that each provide management functionality relating to the storage server system, and a driver that is capable of being associated each of the application clients that are to utilize the data storage system. A data storage configuration identifier structure whose value is updated when there is a change to the composition of the storage system or storage allocation within the storage system is used to manage data transfers between the storage system and application clients.

FIELD OF THE INVENTION

[0001] The present invention relates to data storage and, in particular,to the distribution of data storage over a computer network.

BACKGROUND OF THE INVENTION

[0002] A conventional network computer system is comprised of a numberof computers that each have an operating system, a network forcommunicating data between the computers, and at least one data storagedevice that is attached to at least one of the computers but notdirectly attached to the network. In such a system, the transfer of databetween the data storage device and a computer in the system other thanthe computer with which the device is associated requires that theoperating system of the computer with which the data storage device isassociated to devote a certain amount of time to the processing of thedata transfer. Because the operating system of the computer is typicallyservicing requests from various applications (e.g., a word processingapplication) executing on the computer, the operating system typicallyis only able to devote a limited amount of time to the processing of thedata transfer.

[0003] While data transfer rates over networks were relatively slow, theoperating systems were typically able to service data transfer requestsquickly enough to utilize any available time on the network for datatransfers between computers in the system. In other words, the networks,due to their relatively low transfer rates, were the bottleneck intransferring data between a data storage device associated with onecomputer in the system and other computers in the system. However, asthe data transfer rates for network improved, the operating systembecame the bottleneck because the operating system was typicallyservicing requests from various applications when the network wasavailable for data transfers to or from the data storage device.

[0004] To avoid the operating system bottleneck, data storage deviceswere developed that directly attached to a network, i.e., network datastorage devices. Due to this direct attachment, any computer in thenetworked computer system is able to directly communicate with thenetwork storage device.

[0005] A further advent has been the development of distributed networkdata storage in which there are two or more network data storage devicesare utilized and a mechanism exists for defining a logical volume, i.e.,a unit of data storage, that physically extends over the two or moredata storage devices. Consequently, to computers in a networked computersystem, the logical volume appears to be a single storage device. Anexample of a network computer system that employs distributed networkstorage is comprised of: (a) two fibre channel disk drives; (b) acomputer; and (c) a network for facilitating data transfers between thedrives and the computer. The computer comprises a driver (a program thatallows an operating system to communicate with a device) for each of thedrives and a logical volume manager that controls the drivers so as todefine a logical or virtual volume that extends over the two fibrechannel disk drives.

SUMMARY OF THE INVENTION

[0006] The present invention is directed to a system for use inachieving distributed network data storage in a network and thatprovides the flexibility to achieve additional functionality, such asthe ability to scale the data storage, stripe data, replicate data,migrate data, snapshot data, and provide shared access.

[0007] In one embodiment, the system is comprised of a storage serversystem that is, in turn, comprised of one or more data storage serverswhich provide data storage and data transfer capability for applicationclients in a networked computer system. An application client is acomputer in a networked computer system that is or will execute aparticular application program (e.g., a data base management program)that requires or will likely require data storage and transfercapability. A data storage server is comprised of a data storage device(e.g., a disk drive) and a network interface for communicating, via anetwork, with an application client and a management storage server.

[0008] The system is further comprised of a management storage serversystem that is, in turn, comprised of one or more management storageservers which each provide certain storage management functionalityrelative to any application clients and the storage server system. Amanagement data storage server is comprised of a network interface forcommunicating, via a network, with an application client and the storageservers in the storage system. A management data storage server isfurther comprised of a data storage device (e.g., a disk drive or tapedrive).

[0009] Each of the management storage servers comprises a data storageconfiguration identifier that is used to coordinate the operation of thestorage servers. The value of the identifier is indicative of anallocation of data storage within the storage server system at aparticular point in time. In one embodiment, the value is a time stamp.Other types of values are feasible. The allocation of data storagewithin the storage server system comprises defining any number virtualor logical volumes that are each distributed over one or more of thestorage servers. Each of the management storage servers is capable ofproviding a first value for the identifier to an application client. Forexample, a management storage server provides a first value for theidentifier to an application client as part of the allocation of datastorage to the application client. Further, each of the managementstorage servers is capable of providing an updated value for theidentifier to each of the storage servers after there is a change inallocation of data storage within the storage server system.

[0010] The storage servers use the identifier in deciding whether or notto carry out a data related request from an application client. Toelaborate, a data related request that a storage server receives from anapplication client comprises the most recent value of the data storageconfiguration identifier in the application client's possession. Thestorage server compares the most recent value of the identifier in itspossession to the value of the identifier associated with the receivedrequest. If the values are the same, both the application client and thestorage server understand the data storage allocation to be the same. Inthis case, the storage server proceeds with the processing of the datarelated request. If, however, the value of the identifier in the storageservers possession and the value of the identifier associated with therequest are different, the application client and the storage serverunderstand the data allocation to be different. Stated differently, theapplication client is operating based upon an out of date data storageallocation. In this case, the storage server does not proceed with theprocessing of the request because to do so might corrupt data. In oneembodiment, the storage server causes an error to be generated that isprovided, via the network, to a management storage server. In response,the management storage server provides the application client with anupdated identifier that the application client is then capable ofutilizing to retry the data related requested, if desired.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 is a block diagram of a networked computer system thatemploys an embodiment of the distributed storage system of the presentinvention;

[0012]FIG. 2 is a block diagram of a networked computer system in whichthe application client is a parallel database server and in which anembodiment of the distributed storage system of the present invention isemployed;

[0013]FIG. 3A illustrates the use of bit masks in verify that a page ofdata on one storage server is synchronized with a copy of a page of dataon another storage server when data is being replicated;

[0014]FIG. 3B illustrates the use of bit masks to indicate that a pageof data on one storage server is desynchronized with a copy of a page ofdata on another storage server when data is being replicated;

[0015] FIGS. 4A-4C illustrate an example of the use of a layeringmechanism to migrate data from pages on one volume to pages on anothervolume;

[0016] FIGS. 5A-5C illustrate an example of the use of a layeringmechanism to implement a snapshot operation;

[0017]FIG. 6 illustrates an embodiment of a process implemented by themanagement storage server to manage the storage server system;

[0018]FIG. 7A illustrates an embodiment of a process implemented by thedriver associated with an application client to perform a readoperation; and

[0019]FIG. 7B illustrates an embodiment of a process implemented by thedriver associated with an application client to perform a writeoperation.

DETAILED DESCRIPTION

[0020]FIG. 1 illustrates an embodiment of a networked computer system 10that employs an embodiment of a distributed storage system 12,hereinafter system 12. The networked computer system 10 comprises: (a)an application client system 14 that comprises one or more applicationclients 16 (i.e., a computer that is or will run an applicationprogram); (b) the system 12; and (c) a network 18 for conveyingcommunications between the application clients 16 and the system 12, andbetween elements of the system 12. In the illustrated embodiment, thenetwork 18 is a Gigabit Ethernet network. However, the invention isapplicable or adaptable to other types of networks.

[0021] With continuing reference to FIG. 1, the system 12 is comprisedof a storage system 20 that provides data storage capability to anapplication program executing on an application client. The storagesystem 20 comprises one or more storage servers 22. Each storage server22 comprises at least one data storage device and at least one interfacefor communicating with the network 18. In one embodiment, the datastorage device is a disk drive. However, other types of data storagedevices are feasible. For example, tape drives are feasible. Typically,when the storage server 22 is comprised of multiple data storagedevices, the devices are all of the same type (e.g., disk drives). Itis, however, feasible to use different types of data storage devices.(e.g., disk drives and tape drives, different types of disk drive,different types of tape drives or combinations thereof).

[0022] With continuing reference to FIG. 1, the system 12 is furthercomprised of a management storage server system 24 that providesmanagement functions relating to data transfers between the applicationclients and the storage system 20. The management storage server system24 comprises one or more management storage servers 26. Generally, it isdesirable to have multiple management storage servers 26 for faulttolerance. Each management storage server 26 comprises at least oneinterface for communicating with the network 18 and at least one datastorage device (e.g., disk drive or tape drive). In addition, at leastone of the management storage servers 26 comprises an interface 28 thatallows a user to interact with the server 26 to implement certainfunctionality relating to data transfers between an application client16 and the storage system 20. In the illustrated embodiment, theinterface 28 is a graphical user interface (GUI) that allows a user tointeract with the server 26 via a conventional monitor and keyboard ormouse. Other types of interfaces that communicate with other types ofperipherals (e.g., printers, light pens, voice recognition etc.) ornetwork protocols are feasible. It should also be appreciated that amanagement storage server co-located with a storage server and/ordriver.

[0023] With continuing reference to FIG. 1, the system 12 furthercomprises a driver 29 that is associated each application client 16 andfacilitates communications between the application client 16 and thesystem 12. It should be appreciated that there are alternatives to theuse of driver 29. For example, a Peripheral Component Interconnect (PCI)card or Host Bus Adapter (HBA) card can be utilized.

[0024] Each of the management storage servers 26 comprises a datastorage configuration identifier that relates to a storage configurationmap which reflects composition of the storage system 20 and theallocation of data storage across the storage system 20 to the variousapplication clients 16 at a point in time. The data storageconfiguration identifier has a value that changes when the compositionof the storage system 20 changes or the allocation of storage within thesystem 20 changes. In one embodiment, the value of the identifier is alogical time stamp that monotonically increases as changes occur. Othertypes of logical time stamps are possible. For example, logical timestamps that decrease are possible, as well as logical time stamps whosevalue changes in a predictable manner. Further, time stamps other thanlogical time stamps are feasible. For example, a time stamp thatreflects actual time is also feasible.

[0025] The storage configuration map identifies each of the storageservers 22 in the storage system 20. In addition, the map identifieseach logical or virtual volume, i.e., an amount of data storage that isdistributed between two of more the storage servers 22 that is allocatedto a particular application client 16. Further, the map identifies thepartitioning of each logical or virtual volume, i.e., how much datastorage of the volume is provided by each of the storage servers 22.

[0026] When a management storage server 26 allocates data storage withinthe storage system 20 to an application client 16, the server 26provides an updated value for the data storage configuration identifierto the relevant application client 16 and, more particularly, to thedriver 29 within the application client 16. The identifier is attachedto all requests for data transfers from the storage system 20 by theapplication client. The management storage server 26 also provides eachof the storage servers 22 with the updated value of the identifier. Themanagement storage server 26 may not, however, be able to provide theupdated value to other application clients. Consequently, the otherapplication clients may have outdated values for the identifier thatreflect an outdated configuration.

[0027] The value of the identifier is used by each of the storageservers 22 that receives a request for a data transfer from anapplication client to prevent corruption of the data. To elaborate, eachof the storage servers 22 comprises a comparator that compares the valuefor the identifier that has been most recently received from the amanagement storage server 26 to the value of the identifier appended tothe data transfer request from an application client. If the values arenot equal, then there has been a change in the composition of thestorage system 20 or the allocation of storage within the storage serversystem 20. In this case, since corruption of data could occur orincorrect data could be provided to the application client if thetransfer was carried out, the storage server 22 at least ignores therequest. In one embodiment, the storage server 22 returns an errormessage to the relevant application client or a management storageserver 26 that is processed so as to provide the relevant applicationclient with an updated value for the identifier. Once the relevantapplication client has the current value for the identifier, theapplication client may be able to reinitiate the request for a datatransfer or know that it needs to get the new configuration.

[0028] If the comparator determines that the value for the identifierthat is appended to the request is equal to the value for the identifierthat was most recently provided to the storage server by a managementstorage server, there has been no change in the composition of thestorage system 20 or the allocation of storage within the system 20. Inthis case, the storage server 22 processes the data transfer requestedby the relevant application client.

[0029] Scaling. The system 12 is capable of readily being scaled toincrease or decrease the number of storage servers 22 in the storagesystem 20. To elaborate, a user is able to use the interface 28associated with at least one of the management storage servers 26 topropose a modification to the configuration map that involves either theaddition of a storage server to the storage system 20 or the subtractionof a storage server 22 from the system 20. If there are other managementstorage servers 26 in the management storage server system 24, theproposed modification to the configuration is provided to each of theservers 26. Each of the servers 26 is capable of evaluating the impactof the proposed modification and providing a “vote” indicating approvalor disapproval of the modification. A management storage server mightprovide a disapproving vote if the proposed modification would adverselyaffect the ability to implement certain storage functions. For example,if a management storage server has caused data from an applicationclient to be replicated over two storage servers with a copy on eachserver, the subtraction of one of the storage servers without theaddition of another storage server is likely to be unacceptable. If theproposed change is approved by the management storage servers 26 in themanagement storage server system 24, the configuration map is changed,any re-allocation of storage within the storage system 20 that isrequired by the change is implemented, any copying of data within thestorage system 20 undertaken, and an updated value for the data storageconfiguration identifier is issued to each of the storage servers.

[0030] Striping. The system 12 is capable of implementing striping,i.e., the partitioning of a logical or virtual volume across two or morestorage servers 22. To elaborate, a user is able to use the interface 28associated with at least one of the management storage servers 26 topropose: (a) a logical or virtual volume within the storage system 20for an application client; and (b) the partitioning of such a volumebetween two or more of the storage servers 22 in the storage system 20.The proposed logical volume and proposed partitioning of the volume isprovided to each of the management storage servers 26 for accessing theimpact thereof and providing an approving or disapproving vote. If theproposed logical volume and partitioning thereof is approved by themanagement storage servers 26 in the management storage server system24, the configuration map is changed, any re-allocation of storagewithin the storage system 20 that is required by the change isimplemented, any copying of data within the storage system 20undertaken, and an updated value for the data storage configurationidentifier is issued to each of the storage servers.

[0031] Shared Access. With reference to FIG. 2, an embodiment of anetworked computer system 10′ that comprises the distributed storagesystem 12 and implements shared access is described. The networkedcomputer system 10′ further comprises a particular application clientsystem, namely, a parallel database server system 14′, such as an Oracleparallel database server system. The parallel database server system 14′is comprised of two or more parallel database servers 16′ thatcooperatively operate with one another in the management of a databasethat is or will be stored in a volume on the storage system 20. Theparallel database server system 14′ is further comprised of adistributed lock manager system 30 that is, in turn, comprised of one ormore distributed lock managers 32 that each operate to issue “locks” tothe parallel database servers 16′. A lock relates to a distinct portionof the database that is or will be stored on the volume allocated to theparallel database server system on the storage system 20. The issuanceof a lock to one of the parallel database servers 16′ provides exclusivewrite access or shared read access to the portion of the distinctportion of database to which the lock relates relative to the otherparallel database servers. By providing exclusive write access to onlyone of the parallel database servers 16′, the situation in which two ofthe servers are concurrently updating the same portion of the databaseis prevented.

[0032] It should be appreciated that, while the distributed lockmanagers 30 are illustrated as being separate from the parallel databaseservers 16′, the distributed lock managers 30 are implemented, in oneembodiment, such that each of the distributed lock managers 30 isassociated with one of the parallel database servers 16′. In such anembodiment, each of the distributed lock managers 30 has access to thedriver 29 (via a generic interface associated with the parallel databasemanagement program) that facilitates communication with the distributedstorage system 12. Other implementations of the distributed lockmanagers 30 are feasible, provided each of the lock managers has theability to communicate with at least one of the management storageservers 26.

[0033] Each of the distributed lock managers 30 operates so as tomonitor the parallel database server to which a lock has been issued todetermine if the lock can be returned so that the lock can be issued toanother one of the parallel database servers 16′. In certain situations,a distributed lock manager 30 operates to revoke a lock issued to afirst of the parallel database servers 16. For example, if a distributedlock manager 30 determines that the communication link with the firstparallel database server to which a lock has been issued is no longeractive or available or that the first parallel database server hasfailed, the distributed lock manager 30 revokes the lock issued to thefirst parallel database server. In such a situation, the distributedlock manager 30 can reissue the lock to a second parallel databaseservers.

[0034] A problem with the lock being issued to the second paralleldatabase server is that the first parallel database server, while inpossession of the lock, may have initiated a write request to the volumeon the storage system 20 that was not been processed by the storagesystem 20 by the time the lock had been revoked and issued to the secondparallel database server. This situation occurs if, for example, thewrite request is still traversing the network during the period of timewhen the lock is being revoked and reissued to the second paralleldatabase server. In this case, the possibility exists that the first andsecond parallel database serves could concurrently be updating the sameportion of the volume of the database, a situation that is undesirable.

[0035] To address this problem, one of the distributed lock managers 32communicates, via its driver 29, with one of the management storageservers 26 that a lock is being revoked. In response, the managementstorage server updates a “lock” map. Updating of the “lock” map causesthe value of the data storage configuration identifier to be updated.After the value of the identifier has been updated, the managementstorage server provides the updated value for the data storageconfiguration identifier to each of the storage servers 22 in thestorage system 20. Subsequently, the management storage server issues acommunication to the distributed lock manager that authorizes the lockmanager to reissue the lock.

[0036] Providing an updated value for the data storage configurationidentifier to the storage server 22 prevents the write request that wasinitiated by the first parallel database server from being processed thestorage server. To elaborate, associated with the write request is aparticular value for the data storage configuration identifier that waspreviously provided to the parallel database server by one of themanagement storage servers 26. However, due to the updating of the datastorage configuration identifier, the storage servers 22 have an updatedvalue for the data storage configuration identifier that is differentfrom the value for the identifier associated with the write request.Consequently, if one of the storage server 22 receives the write update,the comparator in the storage server detects the difference in thevalues of the data storage configuration identifiers and, due to thedifference, at least ignores the request for the write update.

[0037] Replication. A user is able to use the interface 28 associatedwith at least one of the management storage servers 26 to cause datafrom an application client to be replicated on the volume of the storagesystem 20 dedicated to the application client such that one copy of thedata resides on one of the storage servers 22 and one or more othercopies of the data each reside on one of the other storage servers 22.This redundancy provides fault tolerance. The user indicates that datais to be replicated by appropriately modifying the configuration map viathe interface 28. Updating the configuration map causes the value of thedata storage configuration identifier to be updated. The updated valuefor the data storage configuration identifier is provided to each of thestorage servers 22 and the driver 29 of application client to which thereplication is relevant. The driver 29 is also provided withconfiguration map or other information that defines the replication thatis to be applied to the application client data, e.g., the relevantvolume and the storage servers on which the copies of the data are toreside.

[0038] A problem with replicating data is that the copies of the datacan become desynchronized, i.e., the copies are no longer identical toone another. For example, copies of data become de-synchronized when afirst copy of the data is updated on one of the storage servers 22 butone of the other storage servers 22 that is to have a second copy of thedata fails before the update occurs on the server.

[0039] This problem is addressed using a bit mask device (also referredto as synchronization bits) in the storage servers on which data is tobe replicated that is, on occasion, interrogated by a management storageserver and used by the management storage server to determine if copieshave become de-synchronized and take remedial action. With reference toFIG. 3A, the operation of the bit mask device is illustrated for thesituation in which copies of a page of data are to be replicated onserver “0” and server “1”. A page of data is a unit of allocation forthe storage system 20, typically on the order of a megabyte in size, butother sizes are feasible. Associated with server “0” is a two bit, bitmask 40 with the first bit of the mask relating to server “0” and thesecond bit relating to server “1”. Associated with server “1” is a twobit, bit mask 42 with a first bit of the mask relating to server “0” andthe second bit relating to server “1”. When the copies of a page of dataon both of the servers are synchronized, the value of each of the bitsin both bit masks is a logical “1”, which is also referred to as a“clean” condition. Whenever the value of each of the bits in both bitmaps is not “1”, then the possibility exists that the copies aredesynchronized. A copy of a page of data is always deemed to besynchronized with itself. Consequently, bit “S0” of the mask 40 isalways set to a logical 1 and bit “S1” of the mask 42 is always set to alogical 1.

[0040] When the driver 29 associated with the application client whosedata is to be replicated issues a write request to server “0”, the writerequest includes clearing bit mask values and restoring mask values. Theclearing bit mask values are the values to which the bits of the bitmask 40 are to be set prior to the processing of the write request byserver “0”. The restoring bit values are the values to which the bits ofthe bit mask 40 are to be set after it is confirmed that the writerequest was processed. The clearing bit mask values are used to updatebit mask 40 prior to processing the write request for server “0”. Oncethe write request for server “0” has been processed by server “0”, theserver issues an acknowledgment with a token to the client application.

[0041] Similarly, the write request issued by the driver 29 to server“1” includes clearing bit mask values and restoring bit mask values. Theclearing bit mask values are used to update bit mask 42 prior toprocessing the write request for server “1”. Once the write request forserver “1” has been processed by server “1”, the server issues anacknowledgment with the token to the client application.

[0042] Once the driver 29 receives acknowledgments from both server “0”and server “1”, the driver 29 includes the token in the next commandsissued to each of the storage servers on which data is being replicated.Typically, the next commands are write requests issued to both server“0” and server “1” to replicate data. The storage server “0” responds toits command by changing the value of the bits in the bit mask 40 to therestoring values, i.e., “11”. The storage server “1” respond to itscommand by changing the value of the bits in bit mask 42 to therestoring values, i.e., “11”. At this point, the value of each of thebits in each of the bit masks 40, 42 is the same, namely, logical “1”.Consequently, the copies of the page of data on server “0” and server“1” are synchronized, i.e., identical to one another.

[0043] With reference to FIG. 3B, a situation in which the bit masks 40,42 are used to identify a situation in which the two copies of the pageof data have become de-synchronized is described. The reason for thede-synchronization is that server “1” was deemed to have failed (i.e.,become unable to process requests or commands) prior to a write requestfrom the client application being issued. As a consequence, when theapplication attempts to replicate the page of data on servers “0” and“1” only the data on server “0” is updated. Consequently, when server“1” is brought back on line, the copy of the page of data on server “1”will be “old” relative to the copy of the page of data on server “0”.

[0044] With continuing reference to FIG. 3B, the copies of the page ofdata on servers “0” and “1” are initially assumed to be insynchronization. As a consequence, the value of each of the bits in bitmasks 40, 42 is the same, namely, a logical “1”. Prior to write requestsbeing issued to servers “0” and “1” to implement a replicationoperation, one of the management storage servers 26 deems server “1” tohave failed. At least one of the management storage servers 26 issues arequest to at least one of the storage servers 22 on occasion todetermine if the storage server is operational. If the server isoperational, the storage server will cause some form of reply oracknowledgment to be sent to the management storage server that issuedthe request within a predetermined amount of time. If a reply oracknowledgment is not received within the predetermined amount of time,the management storage server assumes that the storage server hasfailed. In such a situation, the management storage server updates theconfiguration map, updates the value of the data storage configurationmap identifier, and provides the map and identifier to the applicationclient, as well as the storage servers 22. Since the application clientis aware that server “1” has failed, no write request is issued tostorage server “1”. The write request issued to server “0” includesclearing bit values and restoring bit values. However, due to the changein the storage system 20 caused by the failure of server “1” andreflected in the change in the data storage configuration identifier,the restoring bit values are, unlike in FIG. 3A, set to “10”.

[0045] Server “0”, after receiving the write request but beforeprocessing the write requests, sets the values of the bits in bit mask40 to the clearing bit values, namely, logical “01”. The server thenprocesses the write request and sends an acknowledgment to theapplication client that includes a token. The next command received byserver “0” from the application includes the token. In response, server“0” modifies the bits of the bit mask 40 to the restoring valuesspecified in the restoring bit values that accompanied the writerequest, namely, logical “01”. At this point, since the value of each ofthe bits in bit mask 40, 42 is incapable of being the same value (sincebit mask 40 is set to “10”) the bit masks reflect a de-synchronizationstate. At least one of the management storage servers 26 is monitoringthe bit masks and detects the indication of the copies of the page ofdata being de-synchronized. After the management storage server detectsthis condition, the management storage server typically causes remedialaction to be taken. In this case, the management storage server causethe copy of the page of data on server “0” to be written to server “1”,thereby bringing the copies of the data back into synchronization. Itshould be appreciated that the bit masks are capable of being used todetect de-synchronization that is attributable to other causes.

[0046] The bit mask device described with respect to FIGS. 3A and 3B iscapable of being extended to accommodate a greater number of copies.Further, it should be appreciated that opposite bit values from thosedescribed with respect to FIGS. 3A and 3B can be utilized.

[0047] Migration. A user is able to use the interface 28 associated withat least one of the management storage servers 26 to cause data on onelogical volume to be migrated to another logical volume. This isaccomplished by establishing using a “translucent” layering mechanism.To elaborate, after the user initiates or defines the migration of datathat is to occur, the management storage server saves the portion of thedata storage configuration map that relates to the volume whose datathat is to be migrated (the old volume), identifies this portion of themap as a layer, and orders this layer as a first or old layer. The datastorage configuration map is then updated to reflect the new datastorage configuration and, in particular, to identify the logical volumeto which the data is migrated (the new volume). This causes the value ofthe data storage configuration identifier to be updated. The new map andvalue for the identifier are distributed to the storage servers 22 andto the driver 29 in the relevant application client. In addition, theportion of the configuration map that relates to the new volume to whichthe data is to be migrated is identified as a layer and this layer isordered as a second or new layer.

[0048] After the layering is defined and ordered, data is migrated fromthe old volume to the new volume by two possible mechanisms. First, atleast one of the management storage servers 26 actively monitors each ofthe pages in the first or old layer to determine if the data associatedwith each of the pages in the old volume has not been migrated to thenew volume. If a page is found whose data has not been migrated to thenew volume, the management storage server causes the data from the pageon the old volume to be read, the data to then be written to the newvolume, and the page in the old volume to be marked as “deleted”. Thesecond mechanism for migrating data from the old volume to the newvolume occurs when an application client endeavors to write to a page onthe new volume. In this situation, the driver 29 interrogates the newlayer before issuing the write request relating to the page to determineif the page in the new layer has received the data from thecorresponding page in the old volume. If not, the driver 29 is able to“see through” the “transparent” portion of the new layer that relates tothe page to which data is to be written to the old layer and “see” thatthe data has not yet been migrated from the old volume for thecorresponding page. In this case, driver 29 causes the data from thepage on the old volume to be read, the data to then be written to thenew volume, and the page in the old volume to be marked as “deleted”.Further, after data from the page on the old volume has been migrated tothe new volume, the driver 29 issues the write request that then causesdata to be written to the page on the new volume.

[0049] By marking each page of the old volume as deleted after the datafrom the page has been migrated, a mechanism is provided for preventinga situation that could adversely affect the migration. To elaborate, itis possible for two client applications to be attempting to write to apage in the new volume during the same period of time and when data forthe page has not yet been migrated from the old volume. In thissituation, the driver 29 associated with each application clientendeavors to cause the migration of data from the page on the old volumeto the corresponding page on the new volume. The driver 29 associatedwith one of the application clients will be successful in causing thedata for the page to be migrated and may then cause the data on the pageon the new volume to be updated via a write request. The driver 29associated with the other application client, without the noted marking,would not be aware that the data for the page has been migrated andendeavor to migrate the data to the corresponding page on the newvolume. If this were to happen, the data migrated by the otherapplication client could overwrite the new data established in the pageby the write request issued by the application client that initiallycaused the data to be migrated. To avoid this possibility, driver 29checks the relevant page in the old layer to determine if the data forthe page has already been migrated, before taking any action to migratethe data. If the data for the page has been migrated, then the driver 29aborts the current write request and retries the write request.

[0050] After the data from each page of the old volume has been migratedto the new volume, the old layer is deleted.

[0051] With reference to FIGS. 4A-4C, an example of migration isdescribed. FIG. 4A illustrates an old volume comprised of six pages(0-5) and with data (A-E) in each of the pages and a new volume beforethe migration of any data from the old volume to the new volume. Toeffect the migration, the old volume is further identified as a layerand ordered as the first or old layer. Because data is present in eachof the pages of the old volume at this point, there is no “transparency”associated with the old layer. The new volume is also identified as alayer and ordered as the second or new layer. Because no data is presentin any of the pages of the new volume at this point, there is“transparency” associated with each page in the new layer. This“transparency” allows the driver associated with an application clientto “see” that the data for the page is present in the first or oldlayer.

[0052]FIG. 4B illustrates the old volume and the new volume after thedata (B) in page “1” of the old volume has been migrated to page “1” inthe new volume. At this point, there is no longer any “transparency”associated with page “1” of the new layer, which indicates that the datafrom page “1” in the old volume has been migrated to page “1” in the newvolume. There is still “transparency” associated with the other pages ofthe new layer, which means that the data from the corresponding pages inthe old layer has not yet been migrated. It should also be noted thatpage “1” in the old layer, due to the migration, is now marked asdeleted, which is represented by an “X”.

[0053]FIG. 4C illustrates the old volume and the new volume after thedata for each page of the old volume has been migrated to thecorresponding page in the new volume. At this point, there is no longerany “transparency” associated with the new layer, which indicates thatdata from all of the pages in the old volume has been migrated to thenew volume. Further, each of the pages in the old layer, due to thecompleted migration, is now marked as deleted. As a consequence, the oldlayer is no longer required and can be deleted.

[0054] It should be appreciated that the translucent layering mechanismis capable of being extended to multiple migrations that would requireadditional layers.

[0055] Snapshot. A snapshot preserves the state of a volume at aparticular point in time while also causing the data in the pages of thepreserved volume, the snapshot volume, to be migrated to a new volumewhere the pages can be updated by one of more of the applicationclients. To preserve the state of the snapshot volume, the new volumecannot overlap with the snapshot volume.

[0056] A user is able to use the interface 28 associated with at leastone of the management storage servers 26 to cause a snapshot. Once asnapshot has been initiated, the management storage server 26establishes the same translucent layering mechanism described withrespect to the migration process to facilitate migration of the datafrom the snapshot volume to the new volume. Migration is achieved by themigration of data in a page as a prelude to the issuance of a writerequest from the driver 29 associated with an application. However, incontrast to the migration process, after data for a page is migratedfrom the snapshot volume to the new volume, the page on the snapshotvolume is not marked as deleted. Consequently, the data in the pages ofthe snapshot volume are preserved.

[0057] With reference to FIGS. 5A-5C, an example of snapshot isdescribed. FIG. 5A illustrates a snapshot volume comprised of six pages(0-5) and with data (A-E) in each of the pages and a new volume beforethe migration of any data from the snapshot volume to the new volume. Toeffect the migration, the snapshot volume is further identified as alayer and ordered as the first or old layer. Because data is present ineach of the pages of the snapshot volume at this point, there is no“transparency” associated with the old layer. The new volume is alsoidentified as a layer and ordered as the second or new layer. Because nodata is present in any of the pages of the new volume at this point,there is “transparency” associated with each page in the new layer. This“transparency” allows the driver associated with an application clientto “see” that the data for the page is present in the first or oldlayer.

[0058]FIG. 5B illustrates the snapshot volume and the new volume afterthe data (B) in page “1” of the snapshot volume has been migrated topage “1” in the new volume. At this point, there is no longer any“transparency” associated with page “1” of the new layer, whichindicates that the data from page “1” in the snapshot volume has beenmigrated to page “1” in the new volume. There is still “transparency”associated with the other pages of the new layer, which means that thedata from the corresponding pages in the snapshot layer has not yet beenmigrated. It should also be noted that the data that was in page “1” inthe snapshot volume before the migration is still in page “1” of thesnapshot volume and cannot be altered. The data that has been migratedto page “1” of the new volume is, however, susceptible to modification.

[0059]FIG. 5C illustrates the snapshot volume and the new volume afterthe data for each page of the snapshot volume has been migrated to thecorresponding page in the new volume. At this point, there is no longerany “transparency” associated with the new layer, which indicates thatdata from all of the pages in the old volume has been migrated to thenew volume. Further, it should be noted that the data in each of thepages of the snapshot volume before the migration operation is stillpresent and in the same location after completion of the migration.Hence, the snapshot has preserved the state of the initial volume at aparticular point in time. The data in each of the pages of the snapshotvolume has also been migrated to the new volume and the pages of the newvolume are susceptible to modification as a result of the processing ofwrite requests issued by an application client.

[0060] Management Storage Server Process. With reference to FIG. 6, themanagement storage servers each carry out a process that has two primarytasks: resynchronization of data after a storage server failure orrestart, and the migration of a volume of data. The process has twophases. The first phase involves locating the volumes and pages withinthe volumes that need to be either resynchronized or migrated. Themanagement storage server begins by examining its set of configurationmaps for the volumes currently being managed. From this, the serverdetermines which volumes may require some work because the volume is inthe process of being migrated to a different set of storage servers orbecause at least one of the storage servers storing data for the volumehad failed and then restarted but had not yet been fully resynchronized.After determining the set of volumes requiring work, the managementstorage server then pick one of them, either randomly or according tosome priority. The management storage server then requests that each ofthe storage servers enumerate up to some fixed number of pages thatmatch the migration or resynchronization criteria. The pages areaccumulated by the management storage server with duplicates beingdiscarded. The management then proceeds through the pages, eitherone-by-one or potentially several in parallel, for the second phase ofthe process.

[0061] For each page, the management storage server first requests thestatus of all copies of the page in all the layers associated with thevolume from the associated storage servers. If any of the copies of thepage in any of the layers has synchronization bits that indicate thedifferent copies could contain different data, then these layers of thepage are selected to be resynchronized. They are resynchronized asfollows. The management storage server picks a copy of the page on oneserver which is referred to as the “authoritative copy” and reads thecontents of that copy. The management storage servers must pick theauthoritative copy in such a way that they all pick the same copy asauthoritative. One way to do this is to base the selection oninformation in the configuration map, but other methods are feasible.After reading the authoritative copy, the management storage server thenwrites the contents of the page to the other copies of the page in thatlayer. The management storage server then marks all copies of the pageas being clean by setting their synchronization bits. The managementstorage server is now done with the page for the time being (it ispossible there is still some additional work to be done on the page, butin that case the storage servers will enumerate the page again).

[0062] If no copies of a page need to be resynchronized but there is acopy that needs to be migrated, then the management storage serverfollows these steps. First, the management storage server determineswhich layer will be the source layer and which layer will be thedestination layer. The management storage server then reads one copyfrom the source layer. The management storage server writes that data toall copies of the destination layer. The management storage server thenmarks all the copies on the destination layer clean by setting theirsynchronization bits. Finally, the management storage server requeststhat all copies on the source layer be deleted. At this point, themanagement storage server is done migrating the page.

[0063] Throughout each step of this process, it is possible that astorage server generates an error indicating that the management storageserver is using a value for the data storage configuration identifierthat is out-of-date. If this happens, the management storage server thenrestarts the process. The management storage server also restarts theprocess if any communication errors occur during the process or anyaspect of the configuration map for the volume changes.

[0064] Client Driver Read Process. With reference to FIG. 7A, the driver29 implements a process to read a portion of a page of data for avolume. This process is only initiated after the driver has received acopy of the current configuration map and a value for the data storageconfiguration identifier from a management storage server for the volumethe driver is accessing. The driver starts at the top-most layer andpicks one copy of the page in that layer to read from. The driver maypick the copy to read in any way; including randomly or according to aperformance load metric (trying to pick the least loaded storageserver). If the data exists in that layer, then the driver returns thedata it read to the operating system. Otherwise, the driver advanceslayer by layer, attempting to read the page's data in each layer. If thedriver gets to the last layer without locating any valid copies, thenthe driver returns data to the operating system as though the data werethere but were all zeroes (“0”). If any copy is found to be potentiallyunsynchronized because of the status of the synchronization bits, thenthe driver will resynchronize that data by reading an “authoritativecopy”, writing to all other copies in the layer, setting thesynchronization bits to all-ones (“1”) and then restarting the process.If at any time, a storage server indicates in a reply to a request thatthe configuration value for the data storage configuration identifierthe driver used is old, then the driver requests a new configuration mapfrom a management storage server and restarts the process. The processalso restarts if the management storage server sends the driver a newconfiguration map, if the driver encounters a page that was marked ashaving previously existed but has since been deleted, or if there areany communication errors.

[0065] Driver Write Process. With reference to FIG. 7B, the driver 29implements a process to write data to a portion of a page in a volume.This process is only initiated after the driver has received its firstconfiguration map and data storage configuration identifier from amanagement storage server.

[0066] The process begins by writing the data to all copies of the pagein the top-most or most recent layer. If all writes succeed, then thedriver returns the successful completion to the operating system. If anycopy is not present in the top-most layer, then the driver proceeds toscan down the layers looking for the uppermost copy of the data in allthe layers. If the data is not synchronized, the driver resynchronizesthe data (using the same steps as in the read process above). If thepage is not present in any layers, then zeroes are written to all copiesof the top-most layer, the synchronization bits in all copies are set,and the process restarts. Otherwise, one copy of the data in theuppermost layer is selected, the driver reads the entire page, writesthe driver to all copies in the top-most layer, sets the synchronizationbits in the top-most layer, and then restarts this process.

[0067] As in the other processes, if on any request a storage serverreplies that the driver's configuration ID is old, then the clientdriver requests a new configuration map and data storage configurationidentifier from a management storage server and restarts the process.The process also restarts if the management storage server sends thedriver a new configuration map, if the driver encounters a page that wasmarked as having previously existed but has since been deleted, or ifthere are any communication errors.

1. A system for use in achieving distributed data storage over acomputer network comprising: a storage server system comprising one ormore storage servers that each comprise a data storage device and anetwork interface for communicating with an application client that willrequire data storage and a management storage server; and a managementstorage server system comprising one or more management storage serversthat each comprise a network interface for communicating with anapplication client that will require data storage and each of said oneor more storage servers; wherein each of said management storage serverscomprises a data storage configuration identifier whose value isindicative of an allocation of data storage within said storage serversystem at a point in time; wherein an allocation of data storage withinsaid storage server system comprises defining one or more virtualvolumes of data storage distributed over one or more of said storageservers; wherein each of said management storage servers is capable ofproviding a first value for said data storage configuration identifierto an application client; wherein each of said management storageservers is capable of providing a second value for said data storageconfiguration identifier to each of said storage servers after there isa change in the allocation of data storage within said storage serversystem; wherein each of said storage servers comprises a comparatorcapable of comparing (a) said first value for said data storageconfiguration identifier which is associated with a data storage relatedrequest received from an application client with (b) said second valuefor said data storage configuration and (c) ignoring said data storagerelated request if said first value is not equal to said second value.